The Atlantic is really heating up as we get into August. Two tropical waves over the Atlantic have potential to develop into a tropical cyclone over the next several days. The first one, Invest 99L, is a vigorous, large tropical wave with a broad area of low pressure centered south of the Cabo Verde Islands. The second one, Invest 90L, is a strong tropical wave located over the eastern Caribbean Sea. Both have potential to possibly strengthen and impact land areas, and I think that the Atlantic is likely to see its first hurricane of the season during the next week or so.

Invest 99L: Likely to develop, but how strong will it be and where will it go?

A strong tropical wave emerged off the coast of Africa Thursday morning was designated Invest 99L by the National Hurricane Center (NHC). The NHC gives 99L a 40 percent chance of development within 48 hours, and an 80 percent chance of development within five days. As of 00:00 UTC Friday, 99L was centered near 9.9°N 22.7°W and was moving westward at about 10 to 15 mph. Maximum sustained winds were estimated at 25 knots (30 mph) with an estimated minimum pressure of 1010 mb. 99L is a sprawling wave with plenty of spin. The shower activity extends several hundred miles southwest of Cabo Verde all the way to south-southeast of Cabo Verde. An associated area of low pressure has formed with 99L, but the strongest shower and thunderstorm activity is currently displaced to the west. 99L’s large size may initially inhibit rapid development. Global models appear to indicate that the area of vorticity to the west of the center will eventually become the dominant center of 99L. I think that due to its large size, 99L will take a couple of days to consolidate a dominant center, and likely will not develop into a tropical depression until Saturday at the earliest.

The intensity forecast for 99L is problematic, with mixed signals. Although the most recent GFS, ECMWF, CMC, and UKMET model runs all predicted development, they varied greatly in intensity. The 00Z Friday SHIPS model predicted that throughout the five-day forecast period, 99L will be located over sea surface temperatures of 27-28°C (82.4-84.2°F), with wind shear light to moderate at less than 15 kt. 99L is currently in a very moist environment with mid-level relative humidity values of over 75 percent, but the SHIPS model predicted that 99L would enter a drier environment by Day 4, when relative humidity values are expected to drop to 55-60 percent. The 18z Thursday GFS model run predicted that 99L would develop into a tropical depression by Saturday, and eventually strengthening into a hurricane by Monday. Many GEFS ensemble members also supported the idea that 99L could become a hurricane. This idea of 99L becoming a hurricane is also supported by the HWRF and HMON hurricane models, but these are usually not reliable until a well-defined center of circulation forms. The CMC and UKMET models predicted that 99L would peak as a strong tropical storm.  In contrast, the ECMWF and most EPS ensembles were very weak, mostly depicting a tropical depression or weak tropical storm. I believe the intensity of 99L will likely be largely determined by how much dry Saharan dust it entrains into its circulation. If it takes a southern track, it may be able to avoid the worst SAL. Exact details in the track and intensity forecast are very uncertain as of this point, and will likely become more clear when 99L closes off a single well-defined center of circulation. The first possible land threat for 99L is the Northern Lesser Antilles, which several models bring 99L to in about six to seven days. Residents of the Lesser Antilles should closely monitor the progress of 99L.

Invest 90L could develop in Western Caribbean or Bay of Campeche

A strong tropical wave, located over the southeastern Caribbean Sea, is producing some disorganized shower and thunderstorm activity. This tropical wave was designated Invest 90L by the NHC Thursday morning, and has potential to develop into a tropical cyclone while located over the Western Caribbean or Bay of Campeche. The NHC gives 90L a 20 percent chance of development within 48 hours, and a 40 percent chance of development within five days. As of 00:00 UTC Friday, Invest 90L was centered near 12.3°N 67.3°W, and was moving west-northwestward at about 20 mph. Maximum sustained winds were estimated at 30 knots (35 mph) with an estimated minimum pressure of 1011 mb. The 00Z SHIPS model run predicts that 90L will be located over sea surface temperatures near 28°C (84.2°F), with mid-level relative humidity values above 65 percent and wind shear less than 20 knots for most of the forecast period. The only major hindering factors with 90L’s development are its fairly fast forward speed and proximity to land.

The 12z Friday ECMWF, as well as the 18z Friday GFS model runs, predicted some slight development of 90L by the time it reaches the western Caribbean, with intensification possible if it emerges far enough north into the Bay of Campeche. The 12z Friday UKMET model run did not predict any development, and the 12z Friday CMC model run only predicted a weak area of low pressure in the area. 90L is expected to cross the Yucatan Peninsula by Tuesday, and is likely to emerge into the Bay of Campeche by Wednesday. The GFS is much farther south and predicts that 90L will be a moderate tropical storm making landfall in Veracruz, while the ECMWF is farther north and predicts that 90L will be a Category 1 hurricane making landfall in Northern Mexico. Water temperatures in the Bay of Campeche are very warm – over 31°C (87.8°F), and this could be conducive for rapid intensification if vertical wind shear is low. The quicker 90L develops, the more likely it is to strengthen in the Bay of Campeche or perhaps even the Northwestern Gulf of Mexico if it takes a more northward path than expected. Residents of the Yucatan Peninsula, as well as the eastern Mexican coast and even southern Texas, should closely monitor the progress of 90L.

The next two names on the Atlantic naming list are Franklin and Gert. I will be back tomorrow for an update on 99L and 90L. As stated before, it is too soon to determine the exact track and intensity of 99L and 90L.